It is common practice in vineyards or related agricultural settings to provide a plant-supporting system, sometimes referred to as a trellis, comprising rows of roll-formed steel vertical posts and a number of horizontal cross arms for supporting rows of wires in parallel fashion upon which the agricultural products are trained. Traditional cross arms are fastened to the vertical posts by U-bolts, nuts, washers, or other external hardware. In some cases, the cross arms may be individually welded to the posts. The installation process for attaching the cross arms to the posts is expensive, both from a cost perspective of having to supply separate fastening devices, hardware and tools, as well as the associated time it takes to attach the cross arms to the vertical posts using these fastening methods.